Adhesive composition

ABSTRACT

The invention provides for an adhesive composition that can both be used as melt adhesive composition and as drying adhesive composition. Such an adhesive composition comprises a thermoreversible gelling starch, a bonding starch and a plasticizer.

This application is the U.S. National Phase of, and Applicants claimpriority from, International Application No. PCT/NL2011/050654 filedSep. 27, 2011 and European Patent Application No. 10180979.6 filed Sep.28, 2010, each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention is directed to an aqueous adhesive composition as well asto a method for adhering a first substrate to a second substrate.

Various types of adhesive compositions are known that are suitable foradhering various substrates, such as paper and board.

Adhesives may be divided in groups by their method of adhesion. Forexample, drying adhesives (which can be further subdivided into solutionand emulsion adhesives) and hot melt adhesives are two different groupsof adhesives known in the art.

Drying adhesives set by drying. Drying increases the viscosity of theadhesives and increases the bond strength between two substrates gluedwith the adhesive. Drying adhesives are preferably water-based(waterborne). Drying adhesives can further be subdivided into adhesiveswherein the adhering polymer is dissolved (solution adhesives) andpolymer dispersion adhesives (emulsion adhesives).

Solution adhesives typically comprise hydrophilic polymers, such asnatural polymers (for example starch, dextrin, cellulose derivatives,casein) or soluble synthetic polymers (for example polyvinyl alcohol),which polymers are dissolved in water. As the solvent evaporates, theadhesive sets. Due to their high affinity for water, the polymers in asolution adhesive release water very slowly. As a result, solutionadhesives are generally relatively slow setting and take a longer dryingperiod compared to emulsion adhesives. Solution adhesives areparticularly suitable for the adhesion of porous substrates. Theporosity of the substrate facilitates the diffusion of the solvent fromsolution adhesives, thereby enhancing drying speed.

Emulsion adhesives typically comprise a polymer dispersed in water. Anemulsion adhesive comprises hydrophobic polymers, such as polyvinylacetate (PVAc), dispersed in water. Emulsion adhesives may contain lesssolvent than solution adhesives, because the polymer is not dissolved,but merely dispersed and the water is repelled by the hydrophobic natureof the polymer. As a result, emulsion adhesives generally set fastercompared to solution adhesives. Emulsion adhesives typically have a highsolid content. Emulsion adhesives are frequently used in such industriesas the woodworking and packaging industries.

An advantage of drying adhesives containing non-thermoplastic polymerssuch as natural carbohydrates is that they are not particularlysensitive to temperature. Such drying adhesives are able to provide agood bonding strength between two glued substrates at both low and hightemperatures.

A further advantage of drying adhesives, in particular waterborne dryingadhesives, is that they can often be environmentally friendly.

A further property of drying adhesives is that they set relativelyslowly. This may to a certain degree be advantageous, because it allowsfor a certain assembly time of the two substrates to be glued. However,the setting time of drying adhesives, in particular waterborne dryingadhesives, is typically long and may even be so long that it may giveinsufficient bonding in fast applications. Drying adhesives require oneof the substrates to be water permeable to allow for drying to occur.

A further disadvantage of drying adhesives, in particular waterbornedrying adhesives, is that they are typically vulnerable tomicroorganisms and/or may have poor moisture resistance.

Hotmelt adhesives (hotmelts) are adhesives that are solid at roomtemperature and become a fluid at elevated temperature. When a hotmeltadhesive is heated above its softening point, typically at 140-180° C.,it becomes fluid and can be applied to a substrate. Upon cooling belowthe softening point the hotmelt solidifies and may bind two substratestogether.

Some materials, such as plastics and gels, do not have a definitemelting point. For such materials, a so-called softening point may beused to indicate the material's transition temperature from a solid orgel state to a fluid state. The softening temperature can be determined,for example, by the ring & ball method (ASTM E28-99), the Vicat method(ASTM-D1525 or ISO 306) or the Heat Deflection Test (ASTM-D648). Thesoftening point is preferably determined by the ring & ball method. Thesoftening point as determined by the ring & ball method is thetemperature at which a standardized steel ball cannot be held by a diskof the material anymore. This method of determining the softening pointis described in more detail in the experimental section below.

Hotmelts are typically thermoplastic adhesive compositions. Athermoplastic is an organic material that can be repeatedly softened byheating and hardened by cooling. Hotmelt adhesives are used in manybranches of the adhesive industry, in particular in the packaging andbookbinding industry.

The advantage of hotmelt adhesives is that they typically provide forshort setting times, e.g. below 5 seconds, such that the substrates needonly be pressed together for a short time after glueing (a shortpressing time). At the end of the pressing time, the adhesive strengthshould be high enough to hold the substrates together. Furthermore,hotmelts are able to bond many different types of substrates, includingpermeable substrates and non-permeable substrates.

A disadvantage of hotmelts is that they are inherently temperaturesensitive. Hotmelts become soft at elevated temperatures and becomebrittle at low temperatures. At elevated temperatures hotmelts aresusceptible to joint movement or creep, which can result in bondfailure.

A further disadvantage of hotmelts is that they typically have a highapplication temperature of between 140-180° C. Such high temperaturesmay be undesirable with respect to energy consumption and operatorsafety and may further be undesirable to use in certain applications,for example when one of the substrates to be glued is a heat-sensitivesubstrate.

A further disadvantage of hotmelts is that the maximum amount of timebetween applying the adhesive on one substrate and application of thesecond substrate at which still a good bond is achieved (“open time”) isshort, which makes the time available for part adjustment limited. Partadjustment is the operation of adjusting the two substrates relative toeach other. The operation of part adjustment is possible only for aslong as the glue between the substrates has not fully set yet and thusallows for moving the substrates relative to each other. In order toincrease the open time often more adhesive is applied than needed toachieve a suitable bond strength (see Irving Skeist, Chapter 23:Polyolefin and Ethylene Copolymer-based Hot Melt Adhesives, p 408-p 422,Handbook of Adhesives, Third Edition, 1989, Chapman & Hall). Thissolution is not desirable for obvious reasons.

Plasticizers have been used extensively to obtain hotmelts based onwater soluble polymers, such as starch, which are not inherentlythermoplastic. In particular, low molecular weight, water solublecompounds have been used as plasticizers, such as illustrated in U.S.Pat. No. 5,454,862, WO 92/19690 and GB-A 964 799.

A disadvantage of using plasticizers in such hotmelts is thatplasticizers have a tendency to migrate under the influence of moisture,resulting in bleeding (visible smudging of the surface of the substrate)and bond failure due to brittleness and loss of cohesion.

Although nowadays hotmelts are solvent free systems based on syntheticpolyolefins, the early hotmelt adhesives were based on ethyl celluloseand animal and hide glues and were waterborne. These were abandonedbecause they were suffering from disadvantages such as being unable tocope with higher machine speeds or having too low softening point andbad smell. The thermoplast based hotmelts provided 100% solid adhesivesrequiring no water transport, having no smell, and adhering to morediverse substrates.

U.S. Pat. No. 5,589,528 describes a hot melt adhesive based on modifiedwater bearing gels having a softening point below approximately 43° C.The hot melt adhesive may comprise 50-70% natural organic materialcontent (such as animal hide and bone, fish, blood and casein, soy,starch, cellulosics, etc.), 25-50 wt. % water. The hot melt adhesive mayfurther comprise sugars and tackifiers, for example starch components.

A disadvantage of the composition described in U.S. Pat. No. 5,589,528is that the softening point of the hotmelt is too close to roomtemperature, which will result in slow setting of the adhesive at roomtemperature. A further disadvantage is that the manufacture of thehotmelt is laborious and conducted in multiple steps.

WO 98/15347 is directed to a modified starch obtainable by treatingamylose-containing starch in aqueous medium with an enzyme havingα-1,4-α-1,4-glucosyl transferases activity. Such a modified starch canbe used as an agent for forming a thermoreversible gel. The modifiedstarch is suitable for many uses in which the property to form athermoreversible gel can be useful or important, such as in foodstuffs,cosmetics, pharmaceutics, detergents, adhesives and drilling fluids. WO98/15347 further describes an aqueous solution comprising 5% potatostarch modified with glucosyl transferase, which solution shows thebehaviour of a thermoreversible gel. Disadvantage of the composition ofWO 98/15347 is that its gelling speed is slow. It takes hours for thegel to form, even at low temperatures such as 5° C. This makes thethermoreversible gel of WO 98/15347 unsuitable as an adhesive.

Another approach to obtain a thermoreversible gelling starch is providedin EP 0 372 184. In the process described, starch is partiallydebranched using enzymes with debranching activity. The starchesobtained consist of a mixture of short chain amylose, partiallydebranched amylopectin and amylopectin in ratios depending on startingmaterials and processing conditions. The formation of gels using thesestarches is also too slow to be suitable as an adhesive.

Object of the invention is to provide an adhesive composition that maybe used as a hot melt adhesive composition, which does not have one ormore of the above-mentioned disadvantages of hot melt adhesivecompositions.

A further object of the invention is to provide an adhesive compositionthat may be used as a drying adhesive composition, which does not haveone or more of the above-mentioned disadvantages of drying adhesivecompositions.

A further object of the invention is to provide an adhesive compositionthat can both be used as hot melt adhesive composition and a dryingadhesive composition.

It is a further object to provide an adhesive composition that can notonly be used immediately after preparation while still hot but alsoafter cooling and reheating. Upon cooling, the composition solidifiesand can be easily handled in any form as desired. The solidifiedcomposition can be heated again to regain the liquid compositionprovided excessive evaporation of moisture is avoided.

DESCRIPTION OF THE INVENTION

At least one of the objects was met by providing an aqueous adhesivecomposition comprising a thermoreversible gelling starch and a bondingstarch.

It has surprisingly been found that the presence of a bonding starch inthe adhesive composition may decrease the setting time of thecomposition considerably. It was further surprisingly found that, whilethe presence of the bonding starch decreases the gelling time of thecomposition, it does not or at least not significantly influence thethermoreversible gelling character of the adhesive composition of theinvention in a negative way.

It has further been surprisingly found that the adhesive composition ofthe invention is able to fixate additives, such as plasticizers. This isadvantageous, because it prevents or at least decreases the undesirablerelease of additives from the gel, such as bleeding and other migrationrelated problems found in hotmelts, and bond failure caused by suchundesirable release.

An advantage of the adhesive composition is that it is easy to handle.The adhesive composition of the invention is typically prepared at suchtemperature that the ingredients can be properly mixed and dissolved,for example at a temperature of 50° C. or higher, preferably atemperature of 80° C. or higher. After preparation, the composition maybe cooled and will solidify to a gel. The gel can be easily handled inany form as desired, which makes it easy to transport or store. To usethe gel as an adhesive, it only has to be heated to above its softeningpoint and can then be applied to a substrate.

A further advantage of the adhesive composition is that it is resistantto heat. In case the adhesive composition of the invention is applied toa substrate, the solvent can leave the composition, for example byevaporation or absorption by the substrate. Consequently, the adhesivecomposition may loose its thermoreversible property. This is desirablewith respect to the heat-resistance of the bond between the substrates.Once the two substrates have been adhered together, the bonding strengthbetween the substrates will not significantly decrease upon heating,even when the substrates would be heated above the softening point ofthe initial adhesive composition.

Thus, the composition according to the invention combines the fastsetting of hotmelt adhesives with the beneficial “open times” (seeherein below) and good heat resistance of dissolved starch adhesives.The compositions according to the invention can be used at a temperaturebelow 100° C., which temperature is much safer for the operator thantraditional hotmelt adhesives, which are typically used at temperatureswell above 100° C. However, the adhesive composition of the inventionhas a sufficiently high softening point to facilitate rapid bondformation.

The term “thermoreversible gelling starch” as used herein refers to amodified starch that is capable of forming a thermally reversible gelwhen dispersed in water at a certain concentration. The suitableconcentration to form a thermoreversible gel varies with the compositionof the adhesive composition and the type of thermoreversible gellingstarch used. Thermoreversible gelling starch may for example be obtainedby certain enzymatic treatments of starch, such as described below.

The terms “thermoreversible gel” and “thermally reversible gel” may beused interchangeably and refer to a composition that is a liquid above acertain temperature and forms a gel when cooled below this temperature.The temperature is referred to as the softening point of thethermoreversible gel. The process of going from a liquid to a gel andvice versa is a reversible process. This means that a thermoreversiblegel may be a solution above its softening point, forms a gel uponcooling and after re-heating again above its softening point revertsinto a solution with similar viscosity and clarity as the initialsolution. The term “thermoreversible gel” is an established term in theart, as for example explained in “Food Macromolecules and Colloids, eds.E. Dickinson and D. Lorient, Royal Society of Chemistry, Cambridge,1995, p 480-486”. Accordingly, the adhesive composition of the inventionmay be a thermoreversible gel, in particular when used as a hotmelt.

The term “bonding starch” as used herein refers to a starch that iscapable of strengthening the adhesive bond and increasing the settingspeed of the adhesive composition. The bonding starch may or may notgel, though not thermoreversibly. Thus, the bonding starch is not athermoreversible gelling starch. Consequently, the bonding starch andthermoreversible gelling starch present in the adhesive compositioncannot be the same starch. The bonding starch will increase solidcontent and contribute to bond formation with the substrates.

The term “solid content” as used herein refers to the amount of solidsin the compositions based on the total amount of the composition. Thecompounds that are considered to be solids are typically all compoundsin the adhesive composition other than the solvent and thus includemodified starch, carbohydrates and salts. The non-solid content of thecomposition is typically the solvent, which in most cases only consistsof water.

The term “open time” as used herein refers to the maximum amount of timebetween applying the adhesive on one substrate and application of thesecond substrate at which still a sufficiently strong bond can beformed.

The term “closed time” as used herein refers to the minimum amount oftime it takes to form a sufficiently strong bond after the twosubstrates are applied to each other. During the closed time, the twosubstrates are held together, for example by pressing them against eachother. Therefore, the “closed time” may also be indicated as the“pressing time”. These terms can be used interchangeably.

The term “setting time” as used herein refers to the time it takes foran adhesive composition to achieve a certain bond strength, sufficientto withstand disassembly of the substrates.

The bonding starch is preferably a degraded starch. A degraded starchhas a smaller chain length compared to a starch that is not degraded.The reduction in chain length due to degradation leads to a decrease inviscosity of the starch when dissolved in water. This is advantageous,because the lower viscosity makes it possible to provide an adhesivecomposition with a higher solid content at a given viscosity, and thus ahigher adhesive strength and/or faster setting time. However, on theother hand, a reduction in chain length may also lead to a reduction ofthe adhesive strength because the smaller molecules have less points ofinteraction with the substrates. As a result, there is an optimum in theextent to which the starch should be degraded to obtain the optimaladhesive strength. A skilled person taking the above into considerationwill know how to achieve a suitable adhesive strength.

Suitable ways of degrading starch to obtain bonding starches includeoxidation, e.g. using sodium hypochlorite of hydrogen peroxide asoxidizing agent, acid degradation, dextrinization, heat treatment,extrusion (such as extrusion cooking), or treatment with amylolyticenzymes such as amylases. The degraded starch may further be substitutedto improve the stability of the product, e.g. by acetylation orhydroxyalkylation.

The bonding starch may for example be a maltodextrin preferably having adextrose equivalent (DE) of less than 20, more preferably a DE of lessthan 5. Maltodextrins having such a low dextrose equivalent are referredto as “low DE maltodextrins”. Low DE maltodextrin may be derived fromstarch. Preferably, the low DE maltodextrin is derived from starchcontaining more than 95 wt. % amylopectin, based on the total starchweight. This is advantageous, because these exhibit better stability andbetter adhesive properties due to their higher molecular weight and morebranched fragments. In case the adhesive composition of the invention isused as a hotmelt, the bonding starch is preferably a maltodextrinstarch, in particular a low DE maltodextrin, preferably obtained fromstarch comprising more than 95 wt. % amylopectin.

Preferably, the bonding starch is of medium stability. On the one hand,using a highly unstable starch as the bonding starch is not preferred,because it may result in a loss of thermoreversibility of the adhesivecomposition. Examples of highly unstable starches are fast retrogradingstarches. On the other hand, the bonding starch is preferably not highlystable, because highly stable bonding starches may inhibit gelling ofthe formulation. Examples of highly stable starches arehydroxypropylated starches with elevated degrees of substitution.

The bonding starch does not need to be a gelling starch, i.e. it doesnot need to be capable of forming a gel in an aqueous environmentitself. In case the bonding starch is a gelling starch, it is not athermoreversible gelling starch.

The concentration of the bonding starch in the adhesion composition ispreferably 5-80 wt. %, more preferably 10-75 wt. %, even more preferably30-70 wt. %, based on the total weight of the adhesive composition.

The starch from which the bonding starch is derived may originate fromany suitable source of starch. For example, this starch may originatefrom a botanical source such as root or tuber starches (e.g. potato ortapioca starch), cereal and fruit starches (e.g. maize, rice, wheat orbarley starches), or legume starches (e.g. pea or bean starches).

In one embodiment, the bonding starch is obtained from starch comprisingat least 95 wt. % amylopectin.

Examples of thermoreversible gelling starches are starches that aremodified such that anhydroglucose moieties are transferred from amyloseparts to amylopectin parts of the starch, thus creating longer sidechain amylopectin. Such a thermoreversible gelling starch can beobtained by treating amylose containing starch, typically in aqueousmedium, with an enzyme having α-1,4-α-1,4-glucosyl transferase activity.Examples of such an enzyme are enzymes from the group of theα-1,4-α-1,4-glucosyl transferases, such as amylomaltase. More detailsregarding the treatment of amylose containing starch with an enzymehaving α-1,4-α-1,4-glucosyl transferase activity can be found in WO98/15347.

Another example of a thermoreversible gelling starch is partiallydegraded starch, for example starch comprising up to 80 wt. % shortchain amylose and at least 20 wt. % partially debranched amylopectin.Such starch can be obtained by partially debranching starch by enzymeshaving debranching activity, such as a α-1,6-D-glucanohydrolase.Examples of α-1,6-D-glucanohydrolases are pullulanase and isoamylase.This treatment is described in EP 0 372 184.

A further example of thermoreversible gelling starch is a starchcomprising 5-95 wt. % of a crosslinked starch, e.g. hydroxypropyldistarch phosphate (E1442), and 95-5 wt. % of a degraded starch, e.g.oxidized starch (E1404). Such a thermoreversible gelling starch is forexample known from EP 1,146,795.

EP 0 675 137 describes an adhesive composition comprising a glucanhaving in its molecule one cyclic structure comprising at least 14alpha-1,4-glucosidic bonds. This glucan is not a gelling starch and theadhesive composition described in EP 0 675 137 is therefore not capableof forming a gel, let alone a thermoreversible gel.

The concentration of the thermoreversible gelling starch in the adhesivecomposition is preferably 1-60 wt. %, more preferably 2-40 wt. %, evenmore preferably 3-35 wt. %, based on the total weight of the adhesivecomposition.

The starch from which the thermoreversible gelling starch is derived mayoriginate from any suitable source of starch. For example, this starchmay originate from a botanical source such as root or tuber starches(e.g. potato or tapioca starch), cereal and fruit starches (e.g. maize,rice, wheat or barley starches), or legume starches (e.g. pea or beanstarches).

The adhesive composition may further comprise one or more plasticizers.Plasticizers are widely used to modify the melting properties, tocontrol brittleness and hardness of the hardened adhesive and toregulate the speed of drying. Common plasticizers include glycerine,glycols, sorbitol, glucose, and sugar, in particular sucrose. Thesetypes of plasticizers act as a hygroscopic agent to decrease the dryingrate of the film. Plasticizers based on polyglycols, and sulfonated oilderivates lubricate the layers within the dried adhesive and, thus,impart flexibility. Urea and sodium nitrate plasticize by forming asolid solution with the dried adhesive. Good results have been obtainedusing sorbitol, glycerol, sodium nitrate or sucrose as plasticizer.

The content of the plasticizer in the adhesive composition may be 0.1-50wt %, preferably 1-40 wt. %, more preferably 3-30 wt. %, based on thetotal weight of the adhesive composition.

The adhesive composition preferably comprises a low molecular weightsugar as a plasticizer. Low molecular weight sugars are effective, lowcost, renewable and non-volatile solid plasticizers. The low molecularweight sugar may be selected from the group consisting of saccharose,glucose, fructose, maltose and other oligomeric sugar condensates, aswell as their reduced forms such as sorbitol and mannitol. The contentof the low molecular weight sugar in the adhesive composition may be0-50 wt %, for example 1-15 wt. % or 2-10 wt. %, based on the totalweight of the adhesive composition.

The adhesive composition may have a water content of 20-80 wt. %,preferably 30-70 wt. %, based on the total weight of the adhesivecomposition. Consequently, the adhesive composition may be referred toas an aqueous adhesive composition.

The adhesive composition of the invention is a thermoreversible gel. Thesoftening point of the adhesive composition of the invention istypically higher than 40° C. For compositions requiring cooling, storageand melting prior to application of the glue, a softening point of50-90° C. is preferred. Compositions which are to be used immediatelyafter preparation may have a softening point above 100° C. The softeningpoint of an adhesive composition can for example be increased byincreasing the amount of gelling starch, using a less stable bondingstarch and/or decreasing the amount of plasticizers (such as water orsugar).

The adhesive composition is a viscous liquid above its softening point.Preferably, the viscosity of the adhesive composition measured with aRapid Visco Analyzer as described herein below is 10-30,000 RVU (RapidViscoanalyzer Units), more preferably 100-10,000 RVU, even morepreferably 500-5,000 RVU at 80° C.

The setting time is the time it takes for an adhesive composition toachieve a certain bond strength, sufficient to withstand disassembly ofthe substrates. For example, a sufficient bond strength is considered tobe a bond strength of 40-60 cJ. The setting time required to achievecertain bond strengths may be determined, for example by using thepractical adhesion test described below. The adhesive composition of theinvention may have a setting time of less than 15 seconds to bondstrengths of 40-60 cJ as measured in the practical adhesion test. Thebonding strength between two substrates adhered to each other by anadhesive composition increases as the adhesive composition hardensand/or dries. The setting time depends on many variables, such as thetemperature, the amount of adhesive composition used and the substrateto which the adhesive composition is applied.

The setting time and other properties of an adhesive composition can bemeasured by the practical adhesion test. A standardized amount ofadhesive is applied onto a standardized piece of Kraft paper. Kraftpaper is paper produced from the chemical pulp of softwood processed bythe Kraft process. It is strong and relatively coarse and usually has agrammage of 50-135 g/cm². After applying the adhesive to the first pieceof Kraft paper, a second piece of Kraft paper is placed on top of thefirst and the two pieces of Kraft paper are pressed together. The timebetween applying the adhesive and placing the second paper (the opentime) may vary. After a certain period of time wherein the adhesive isallowed to set (closed time), the strength of the bond formed betweenthe papers is assessed by determination of the amount of work needed topull the two pieces of Kraft paper apart. Specific ways in which thepractical adhesion test may be conducted are described below in the‘Experimental Procedures’ section.

The amount of work needed to pull the two pieces of Kraft paper apart(bonding strength) at a certain open time-closed time interval will behigher for compositions with shorter setting time (if all otherconditions are kept the same).

The initial bonding strength may be determined by measuring the forceneeded to overcome the bond strength between the pieces of Kraft paperat a closed time of 0, 3 and 5 seconds. If this force is relativelyhigh, e.g. higher than 40 cJoule, this is an indication that the bondcan already withstand certain stress although it is not fully set yet.

At forces of 60 cJoule and higher, fibre tear and cohesive failure ofthe adherents are observed for most adhesives (unless they are pressuresensitive), which indicates that the bond can withstand full stress.Therefore, the closed time at 40 and 60 cJoule can be suitably used as avalue to compare the setting time of different adhesive compositions.The setting time also depends on the amount of adhesive used, so thelayer thickness needs to be specified to compare adhesives on fairplaying ground.

For adhesive compositions based on starches, which are adhesivecompositions comprising starch as the main bonding polymer, settingtimes are typically 30 seconds or more as measured by the practicaladhesion test, using 60 μm of adhesive layer. Starch is considered to bethe main bonding polymer in an adhesive composition when at least 50 wt.% of the total bonding polymer weight in the composition is starch,preferably at least 80 wt. %.

The adhesive composition of the invention may further comprise one ormore additives.

The adhesive composition may for example comprise one or more additivesselected from the group consisting of anti-foaming agents, tackifiers,rheology modifiers, fillers, anti-oxidants, water resistance modulatorsand preservatives.

The adhesive composition may further comprise a liquefier, for example aliquefier selected from the group of compounds comprising calciumchloride, urea, sodium nitrate, thiourea and guanidine salts. Liquefiersmay reduce viscosity or may act as humectants to control open time andspeed of drying.

The adhesive composition may further comprise a colloid stabilizer, suchas soaps and sodium chloride, to retard the tendency of retrogradation.

The adhesive composition may further comprise a compound to increase thewater resistance of the composition, such as polyvinyl alcohol,polyvinyl acetate blends, and thermosetting resins, such as ureaformaldehyde or resorcinol formaldehyde.

The adhesive composition may further comprise mineral fillers, such askaolin clay, calcium carbonate and titanium dioxide. Mineral fillers maybe used at concentrations of 5-50%. These are used to reduce cost andcontrol penetration into porous substrates.

The adhesive composition may further comprise a thixotropic agent, suchas bentonite. Thixotropic agents may be used to control rheologicalproperties.

The adhesive composition may further comprise additives such aspreservatives, bleaches, and defoamers.

Suitable descriptions of starch and dextrin based adhesives can be foundin Edward M. Petrie, Starch and Dextrin Based Adhesives, SpecialChem—May12, 2004; Irving Skeist, Chapter 8: Starch Based Adhesives, p 153-165,Handbook of Adhesives, Third Edition, 1989, Chapman & Hall or Lazarus,D. M., “Adhesives Based on Starch”, Chapter 10, Adhesives and Adhesion,Vol. 7, K. W. Allen, ed., Applied Science, London, 1983.

The invention is further directed to a method for adhering a firstsubstrate to a second substrate comprising the steps of

-   -   providing an aqueous adhesive composition comprising a        thermoreversible gelling starch; and    -   connecting the first and the second substrate using the adhesive        composition, wherein the adhesive composition is in liquid form;        and    -   cooling the adhesive composition, for example to a temperature        below its softening point.

As described above, U.S. Pat. No. 5,589,528 describes a hot meltadhesive based on modified water bearing gels having a softening pointbelow approximately 43° C. A disadvantage of this composition is thatthe softening point of the hotmelt is too close to room temperature.This may be disadvantageous in applications wherein substrates are boundat temperatures higher than 43° C., or even at 20° C., where the smalltemperature gap results in slow cooling and thus setting of the gel.

The method of the invention provides for adhering a first substrate to asecond substrate using an adhesive composition having a softening pointbetween 44 and 100° C. This was achieved by using an adhesivecomposition comprising a thermoreversible gelling starch and a bondingstarch, in particular an adhesive composition comprising athermoreversible gelling starch, a bonding starch and a plasticizer.Such starches are further described hereinabove. The adhesivecomposition used in the method of the invention may be an adhesivecomposition of the invention, as described hereinabove.

As already described above, the adhesive composition of the invention istypically prepared at such a temperature that the ingredients can beproperly mixed and dissolved, for example at a temperature of 50° C. orhigher, preferably a temperature of 80° C. or higher. Connecting thefirst and the second substrate using the adhesive composition istypically conducted with the liquid adhesive composition having atemperature of 50° C. or higher, preferably a temperature of 80° C. orhigher.

The adhesive composition in liquid form in the second step may befreshly prepared. However, it may also be prepared from an adhesivecomposition in gel form that was prepared earlier and for exampletemporarily transported and/or stored in gel form.

The adhesive composition may be cooled to a temperature of 40° C. orlower. In particular, the adhesive composition is cooled to atemperature around room temperature, for example to a temperaturebetween 0 and 30° C.

The substrates may be polar substrates, such as paper, cardboard orwood, or non-polar substrates. Preferred are substrates where at leastone is water-permeable. This has the advantage that water leaves theadhesive composition upon setting, which makes the assembly moreresistant to heat.

The invention will be further illustrated by the following examples.

EXPERIMENTAL PROCEDURES

Preparation of Basic Formulation

A closed, 2 liter steel laboratory cooking vessel (having an internaldiameter of 13.5 cm and a height of 15.5 cm measured on the inside ofthe vessel) with jacket heating, fitted with a matching anchor stirreris put on a heating plate and the jacket is filled with cold water. Theanchor stirrer has a width of 12 cm measured at its bottom and comprisestwo anchor arms parallel to the rotation axis of the stirrer, which armshave an internal diameter of 0.9 cm and a height of 9 cm. The liquidcomponents (i.e. water and any liquid or dissolved additives) are addedand a mixture of the solid components (i.e. the bonding starch, thegelling starch and any solid additives) is added while stirring at 150rpm. The total amount of composition was between 400 and 800 g. Theheating plate is turned on to maximum temperature (300° C.) and thesolution is heated, whilst stirring at maximum 150 rpm or lower if theviscosity of the solutions makes this necessary, until the water in thejacket starts to boil. The jacket is flushed with water for 15 secondsand the heating plate is turned off. Additionally the solution isstirred for 20 minutes at 150 rpm. Next the jacket is flushed for 15seconds again and the adhesive is poured into two glass jars which arethen closed with a well fitting lid. One glass jar is stored in an ovenof 100° C. to determine the adhesive properties directly afterpreparation, the other glass jar is stored at room temperature for atleast one day to determine the solidifying and melting properties aswell as the adhesive properties after re-melting.

Description of Measurements

RVA Viscosity—

With a Rapid Visco Analyser (RVA, type 4) the viscosity and theviscosity temperature profile of the prepared adhesive is determineddirectly after preparation and/or after re-melting. In the standard oneway disposable cup 28 gram of the hot liquid adhesive is added.Subsequently the cup is fitted with a standard paddle, inserted into theRVA and the viscosity in RVU (Rapid Viscoanalyzer Units) is determinedaccording to the following profile:

-   -   start temperature 95° C.,    -   heating for 2 minutes at 95° C. without stirring,    -   heating for 3 minutes at 95° C. whilst stirring with 50 rpm.    -   cooling down, whilst stirring with 50 rpm, with 2° C. per minute        till 10° C. The viscosity recorded at 80° C. is given in Tables        1-4.    -   keeping at 10° C., whilst stirring with 50 rpm, for 5 minutes.        The test is stopped automatically if the viscosity becomes        higher than 70000 RVU.

Practical Adhesion (Setting Time)—

The practical adhesion is determined according to the pulling testmethod that is performed with a Fipago-Adhesion tester (system PKL). Thetest is performed in a conditioning room capable of maintaining arelative humidity of 50±2% at 23±1° C. The paper adherents are storedunder the same conditions. The adhesive is tested 1-3 hours afterpreparation and after re-melting at least 1 day after preparation. Theadhesive tested has a temperature of 80±5° C.

A thin film (standard 60 μm) of the hot solution is applied on the sieveside of a standard Kraft paper sample (dimensions: 70 g/m², 200×30 mm)by means of a wire winded rod which is also heated to the sametemperature as the adhesive (80±5° C.). Immediately the glued paper isplaced on top of another Kraft paper (dimensions: 200 g/m², e.g. 100×55mm) and adhered (felt side).

In particular, the application of the hot solution on Kraft paper andgluing it to another Kraft paper may be conducted as follows: a thinfilm (standard 60 μm) of the hot solution may be applied on the smoothside of standard Kraft paper [Natural machine-glazed Kraft paper (onesmooth and shiny side, one matt side) Manufacturer: Sopal Doetinchem,The Netherlands; Gurley porosity: 72 s; PPS smoothness (smooth side):3.42 μm; Cobb⁶⁰: 24 g/m²; Grammage: 85 g/m²; Dennison wax test: 18;dimensions 30×200 mm] by means of a wire winded rod which is also heatedto the same temperature as the adhesive (80±5° C.), after which theglued paper is immediately adhered on top of another Kraft paper [PiteaRoyal Brown, Manufacturer: Kappa Smurfit, Sweden; Supplier: Fipago, TheNetherlands (Fipago 2006 kraftline); Grammage: 200 g/m²; Cobb¹⁸⁰⁰: 86g/m²; Dennison wax test: 18; dimensions: 60×100 mm] by means of a metalpressure roller (500 gram).

The open time in this experiment is set at 0 seconds. The closed time isvaried standard in the interval 0-20 seconds but can be longer if fibretear has not yet occurred. Every adhesive is characterized by at leastfive different closed times, yielding a more or less sigmoid curve. Thiscurve represents the work needed to overcome the bond strength asfunction of closed time. Results are given as work in cJ at [x,y], wherex is the open time used in seconds, and y the closed time in seconds.The pieces of Kraft paper are examined after the measurement to evaluatethe quality of the bond formation, in particular adhesive transfer tothe second piece of Kraft paper.

If the adhesive transfer to the second substrate is insufficient,indicating a crossing of the open time, then the amount of adhesiveapplied can be raised by using a different wire winded rod. Subsequentlythe amount of adhesive applied can also be lowered, by using a differentwire winded rod, to determine the relation adhesive amount/setting timeor adhesive amount/open time.

Practical Adhesion (Wet Tack)—

The wet tack is determined according to the pulling test method that isperformed with a Fipago-Adhesion tester (system PKL). The test isperformed in a conditioning room capable of maintaining a relativehumidity of 50±2% at 23±1° C. The paper adherents are stored under thesame conditions. The adhesive is tested 1-3 hours after preparation andafter re-melting at least 1 day after preparation. The adhesive testedhas a temperature of 80±5° C. A thin film (standard 60 μm) of the hotsolution is applied on the on the smooth side of standard Kraft paper[Natural machine-glazed Kraft paper (one smooth and shiny side, one mattside) Manufacturer: Sopal Doetinchem, The Netherlands; Gurley porosity:72 s; PPS smoothness (smooth side): 3.42 μm; Cobb⁶⁰: 24 g/m²; Grammage:85 g/m²; Dennison wax test: 18; dimensions 30×200 mm] by means of a wirewinded rod which is also heated to the same temperature as the adhesive(80±5° C.).

After the open time is exceeded (standard 5 seconds for wet tack) theglued paper is adhered on top of another Kraft paper [Pitea Royal Brown,Manufacturer: Kappa Smurfit, Sweden; Supplier: Fipago, The Netherlands(Fipago 2006 kraftline); Grammage: 200 g/m²; Cobb¹⁸⁰⁰: 86 g/m²; Dennisonwax test: 18; dimensions: 60×100 mm] by means of a metal pressure roller(500 gram). The closed time is set at 3 seconds. Results are given aswork in cJ at [5,3], where 5 is the open time used in seconds, and 3 theclosed time in seconds.

The pieces of Kraft paper are examined after the measurement to evaluatethe quality of the bond formation, in particular adhesive transfer tothe second piece of Kraft paper. If the adhesive transfer to the secondsubstrate is insufficient, indicating a crossing of the open time, thenthe amount of adhesive applied can be raised by using a different wirewinded rod. Subsequently the amount of adhesive applied can also belowered, by using a different wire winded rod, to determine the relationadhesive amount/wet tack or adhesive amount/open time.

Dry Solids—

The concentration of the adhesive is determined directly afterpreparation by means of a refractometer (Atago AX-1000), and presentedas a brix value.

Softening Point (Ring & Ball or R&B)—

In the standard R& B apparatus, which is described in e.g. ASTM E-28-99(2009), rings and steel balls are used to determine the softening point.The ring-and-ball softening point is defined as the temperature at whicha disk of the sample held within a horizontal ring is forced downward adistance of 1 inch (25.4 mm) under the weight of a steel ball as thesample is heated at a certain rate in a glycerol bath. The ring isplaced on a small glass plate (size: 10×6×0.35 cm) and filled withfreshly prepared hot liquid adhesive avoiding entrapment of air bubbles.A second small glass plate is put directly after filling on the ring.The filled ring is stored between the two glass plates for at least 24hours. The filled ring is taken out of the two glass plates and putimmediately in the R& B apparatus. The steel ball is put on the filledring and the apparatus is then placed in a beaker glass with stirringbar filled with glycerol at 25±5° C. The beaker with the apparatus isput on a heating device with magnetic stirring. A digital thermometer isput in the apparatus. The temperature of the heating device is adjustedsuch that the average temperature increase is 4° C. per minute. Themagnetic stirring is adjusted such that a good circulation of theglycerol, leading to an evenly divided temperature, is assured. Thetemperature at which the steel ball touches the second horizontal plateof the apparatus is recorded as the R&B softening point.

Heat Resistance—

Water resistant 120 grams Kraft paper (20×25 cm) is conditioned at least24 hours at a relative humidity of 50±2% and temperature of 23±1° C. Apiece of Kraft paper is put on top of a glass plate (size: 40×30×0.35cm). Onto the Kraft paper a line of the hot liquid adhesive is poured inan amount of about 6 g/m, about 5 cm from the bottom of the long side ofthe paper. A second piece of the Kraft paper is put on immediately,followed by a second glass plate. Subsequently, a weight of 10 kg isplaced on top of the glass plate for 10 minutes. After these 10 minutes,the conglutinated papers are conditioned for at least 24 hours at arelative humidity of 50±2% and temperature of 23±1° C.

A second set made in the same manner using an adhesive line of about 11g/m.

The conglutinated papers are cut into strips of 3 cm wide, perpendicularto the adhesive line, the two outer strips are not used. The ends of thestrips are folded back and stapled to the strip just above the gluedarea to form loops.

Six strips per test bond are hung from the ceiling of an oven by oneloop and to the other loop a weight of 200 g is attached. Thetemperature of the oven is set at 30° C. and after 10 minutes the stripsevaluated and checked whether the bonds hold. The temperature is raisedto 35° and after 10 minutes checked again. This procedure is repeateduntil 100° is reached and hereafter the temperature is raised byincrements of 10° until 200°.

The temperature, at which a bond breaks, is recorded. The lowest andhighest values are discarded and the heat resistance is the mean of theremaining four.

Re-Melting—

The adhesive in the glass jar that was stored at room temperature for atleast 24 hours is put in an oven of 100° C. If re-melting occurs, theappearance, RVA viscosity and setting time are determined.

Example 1 Preparation of Adhesive Compositions

In table 1, three recipes according to the invention have been preparedfollowing the procedure given above. Eliane MD2 is a maltodextrin withDE of about 2 based on waxy potato starch and functions as bondingstarch, Etenia 457 is an amylomaltase treated, thermoreversible gellingpotato starch, both products from AVEBE. Synperonic L61 (from UniqemaChemie) is an anti-foam agent. Product 1 has a setting time to 40 cJ ofjust over 3 seconds, and of 10 seconds to 60 cJ. Replacing part of thesugar (i.e. sucrose) and gelling starch with bonding starch results infaster setting times (compare recipes 1 and 2). All products solidifyupon cooling. Lowering the amount of gelling starch lowers the softeningpoint (R&B) to below 100 C, enabling product 3 to melt again to anadhesive with properties very similar to the original properties(compare recipes 3 with recipes 1 and 2).

TABLE 1 Recipe number 1 2 3 Water (total quantity before cooking)  48.55   48.55 48.55 ELIANE MD 2 (g dry substance) —   13.30 42.70ETENIA 457 (g dry substance)   33.60   33.60 4.20 Sugar (sucrose) (g drysubstance)   17.80    4.50 4.50 Synperonic L 61 (g as is)    0.05   0.05 0.05 Directly after preparation Viscosity 80° C. [RVU] 2690 3553  736 Refraction [%]   54.4   55.7 55.1 Adhesive amount: 60 μm Wettack [cJ] [5,3]   21.5*  45* 36.5 Adhesion Work at [0, 0] [cJ]  30*  48*21 Work at [0, 3] [cJ]  39*   56.5* 38 Work at [0, 5] [cJ]  45*  73* 50Setting time till 40 cJ [s] 3-4  0 3-4 Setting time till 60 cJ [s] 10  48 Adhesive amount: 200 μm Wet tack [cJ] [5, 3] 54   65.5 42 AdhesionWork at [0, 0] [cJ]   43.5 60 24 Work at [0, 3] [cJ] 51 68 37 Work at[0, 5] [cJ] 64 80 46 Setting time till 40 cJ [s]  0  0 3-4 Setting timetill 60 cJ [s] 4-5 2-3  ±7-8** R & B [° C.]  117.6  114.5 81.2Appearance after 24 hours at room Hard gel Hard gel Hard gel temperatureAfter heating at 100° C. Appearance Hard gel Hard gel Fluid Viscosity80° C. [RVU] Not fluid Not fluid 3007 Wet tack [cJ] [5, 3] 30.5*Adhesion Work at [0, 0] [cJ] 20 Work at [0, 3] [cJ] 35 Work at [0, 5][cJ] 46 Setting time till 40 cJ [s] 4-5 Setting time till 60 cJ [s] 9*Diminished adhesive transfer **Indication (no measurement to 60 cJ)

Example 2 Effect of Thermoreversible Gelling Starch Type

In Table 2 (see next page) the type of thermoreversible gelling starchis varied. It is shown that instead of Etenia 457, Perfectagel MPT orGel ‘n’Melt can be used. Perfectagel MPT is a modified potato starchcontaining hydroxypropyl distarch phosphate (E1442) and oxidized potatostarch (E1404). It is a product from AVEBE often used in meltableimitation cheese. Gel ‘n’Melt is a partially debranched waxy corn starchfrom National Starch.

Example 3

The compositions according to the invention have an open time which ismuch longer than synthetic hotmelts known in the art. This isillustrated by the adhesive transfer in the practical adhesion test asgiven in Table 3, which improves with layer thickness (a thicker layercools slower). Also given are adhesion strengths at [0,3] and [5,3]. Ifthe open time of the adhesive is less than 5 seconds, there is limitedadhesive transfer, and the strength at [5,3] is less than [0,3]. Theincreased open time facilitates reduction of the adhesive layerthickness, and thus reduction of amount of adhesive needed to bond thesubstrates. Two formulations according to the invention are compared toa commercial packaging hotmelt, Swift B569/38 from Forbo Adhesives. Thecommercial hotmelt has an open time which is shorter than the timeneeded to apply the glue onto the Kraft paper and add the second layerof Kraft paper in the test. Furthermore the heat resistance is improvedtremendously with the formulations according to the invention comparedto the synthetic hotmelt.

TABLE 2 Recipe number 4 5 6 Gelling starch used ETENIA PERFECTAGEL GEL‘N’ 457  MPT MELT Water (g added) 50 50 50 ELIANE MD 2 (g as is) 50 5050 Gelling starch (g as is)  5  5  5 Sugar (sucrose) (g as is)  5  5  5Synperonic L 61 (g as is)    0.05    0.05    0.05 Directly afterpreparation Viscosity 80° C. [RVU] 1127  2111  1089  Refraction [%]  55.4   56.3   55.1 Wet tack [cJ] [5, 3]  43*  46*  40* Adhesion Workat [0, 0] [cJ] 29 29 29 Work at [0, 3] [cJ] 48 50 44 Work at [0, 5] [cJ]56 54 60 Setting time till 40 cJ [s] 2-3 2-3 2-3 Setting time till 60 cJ[s] 6-7 6-7  5 Appearance after 24 Hard gel Hard gel Hard gel hours atroom temperature After heating at 100° C. Appearance Fluid Fluid FluidViscosity 80° C. [RVU] 1446  2866  1807  Wet tack [cJ] [5, 3]  43*  41* 37* Adhesion Work at [0, 0] [cJ]   29.5 33 28 Work at [0, 3] [cJ] 43 4342 Work at [0, 5] [cJ] 53 60 56 Setting time till 40 cJ [s] 2-3 2-3 2-3Setting time till 60 cJ [s] 6-7  5  6 *Diminished adhesive transfer

TABLE 3 Synthetic Recipe hotmelts 7 8 Swift B 569/38 Water (total beforecooking) 48.55 48.55 ELIANE MD 2 (g dry) 42.70 13.30 ETENIA 457 (g dry)4.20 33.60 Sugar (sucrose) (g dry) 4.50 4.50 Synperonic L 61 (g as is)0.05 0.05 Directly after preparation Viscosity 80° C. [RVU] 719 57551300@ Refraction [%] 55.8 55.4 Not relevant Adhesive layer: 20 μm Wettack [cJ] [0,3] 47.5 70 Adhesive transfer [0,3] Moderate Poor Wet tack(cJ)[5, 3] 19 55 Adhesive transfer [5,3] Poor Poor Adhesive layer: 40 μmWet tack [cJ] [0,3] 53 77 Adhesive transfer [0,3] Good Moderate Wet tack(cJ)[5, 3] 31 66 Adhesive transfer [5,3] Moderate Poor Adhesive layer:60 μm Wet tack [cJ] [0,3] 47.5 71 Adhesive transfer [0,3] Good ModerateWet tack (cJ)[5, 3] 42 57 Adhesive transfer [5,3] Good Poor Adhesivelayer: 80 μm Wet tack [cJ] [0,3] 43.5 65 Adhesive transfer [0,3] GoodGood Wet tack (cJ)[5, 3] 41.5 56.5 Adhesive transfer [5,3] Good ModerateAdhesive layer: 200 μm Wet tack [cJ] [0,3] 37 67.5 Adhesive transfer[0,3] Good Good None Wet tack (cJ)[5, 3] 42 65.5 Adhesive transfer [5,3]Good Good None R & B [° C.] 81.2 114.5  84.9 Heat resistance [° C.]Adhesive amount 6 g/m >180 >180  45 Adhesive amount 11 g/m >170 >170 47.5 @Specified in product technical data sheet, at 160° C.

Example 4

In the Table 4 a number of bonding starches are used in adhesivecompositions according to the invention using the composition of Example2 but cooked with less water evaporation resulting in a slightly lowersolids content. The waxy potato maltodextrin Eliane MD2 (recipe 9) givesthe best combination of setting speed and melting behaviour. The waxypotato starch based dextrin B2128 gives comparable properties to ElianeMD2 (recipe 10), though it is slightly more difficult to re-melt. Usingan acid degraded, unstable bonding starch like MS 1168 (recipe 11)increases the setting speed but diminishes the meltability of thecomposition significantly. The use of Solfarex A55, an oxydized,hydroxyethylated (degree of substitution 0.10-0.12) potato starchresults in a composition as fast as MS1168 yet as meltable as B2128(recipe 12).

Preparation of B 2128

In a Hobart mixer, 500 g (400 g dry matter) of amylopectin potato starch(amylopectin content>98 wt. %) was mixed with 71 g solution of 0.28 g ofHCl in water for 30 minutes. The mixture was equilibrated at 4-8° C. for16 hours and dried to 4% moisture in a Retsch fluid bed dryer in threesteps of 15 minutes with temperatures of 60, 60 and 90° C. respectively.The mixture was subsequently heated at 90° C. in a rotating reactor for1 hour yielding an off white powder with a moisture content of 3%.

Preparation of MS 1168

In a beaker of 2 liter, 472 g (405 g dry matter) of amylopectin potatostarch (amylopectin content>98 wt. %) was suspended in water to aconcentration of 39 wt. %.

The mixture was heated to 45° C. and then 50 ml 10 N H2SO4 was added.The mixture was stirred at 45° C. for 17 hours and afterwards themixture was dewatered and washed with 2.5 liter water. After drying in astove at 35° C. for 1 night the product was grinded to a powder.

TABLE 4 Recipe 9 10 11 12 Starch used ELIANE B 2128 MS 1168 SOLFAREX MD2 A 55 Directly after preparation Viscosity 80° C. 666 673 2217 1107[RVU] Refraction [%] 53.8 52.6 53.5 51.4 Wet tack [cJ] [5,3] 35 35 38 48Adhesion Work at [0, 0] [cJ] 18 16 38.5 26.5 Work at [0, 3] [cJ] 32 3546 50 Work at [0, 5] [cJ] 41.5 42 55 61 Setting time till 40 cJ 5 5 0-32 [s] Setting time till 60 cJ 9-10 9 6-7 5 [s] Appearance after 24 Hardgel Hard gel Hard gel Hard gel hours at room temperature After heatingat 100° C. Appearance Fluid Just Just Just fluid fluid fluid Viscosity80° C. 1157 3906 5167 3715 [RVU] Wet tack [cJ] [5,3] 32 31 50 47Adhesion Work at [0, 0] [cJ] 14 18 39 29 Work at [0, 3] [cJ] 28.5 31 5447.5 Work at [0, 5] [cJ] 38 38 63 57 Setting time till 40 cJ 5-6 5-7 0-32 [s] Setting time till 60 cJ 10-11 10-12 5 6 [s]

Example 5 Effect of Plasticizer Type

In Table 5 the type of plasticizer is varied. As gelling starch ETENIA457 (see Example 1) and as bonding starch MS 1168 from Example 4 isused. To improve the meltability a formulation with dry solids lowerthan in Table 4 is used. It is shown that besides Sucrose also Sorbitol,Glycerol or sodium nitrate can be used. Sorbitol is D-Sorbitol (fromProlabo), Glycerol is EMPROVE exp (from Merck), Sodium nitrate is sodiumnitrate p.a. (from Merck).

TABLE 5 Recipe 13 14 15 16 Plasticizer used Sugar Sorbitol GlycerolSodium (sucrose) nitrate Water (g added)   53.5   53.5 53.5 53.5 ETENIA457 (on dry   4.1   4.1 4.1 4.1 solids) MS 1168 (on dry solids)   37.9  37.9 37.9 37.9 Plasticizer (g as is)   4.5   4.5 4.5 4.5 Directlyafter preparation Viscosity 80° C. [RVU] 1772  1478  1315 1438Refraction [%]   50.7   50.5 50.0 49.9 Wet tack [cJ] [5,3] 42 42 41 47Adhesion Work at [0, 0] [cJ] 34 34 30 34 Work at [0, 3] [cJ] 46 43 42 42Work at [0, 5] [cJ]   51.5 50 48 54 Setting time till 40 cJ [s] 2-3 2-32-3 2-3 Setting time till 60 cJ [s] 7-8 8-9 8-9 7-8 Appearance afterHard gel Hard gel Hard gel Hard gel 24 hours at room temperature Afterheating at 100° C. Appearance Liquid Liquid Liquid Liquid Viscosity 80°C. [RVU] 2107  1706  1840 2254 Wet tack [cJ] [5,3]  35*  36* 36 40Adhesion Work at [0, 0] [cJ]   31.5 29 28 27 Work at [0, 3] [cJ]   47.542 36 41.5 Work at [0, 5] [cJ] 57   46.5 44 48 Setting time till 40 cJ[s] 2-3 2-3 4 2-3 Setting time till 60 cJ [s] 6-7 8-9 9-10 8-9*Diminished adhesive transfer

Example 6

In table 6 the effect of the amount of plasticizer in the formulation isillustrated. As gelling starch ETENIA 457, as bonding starch MS 1168from example 4 and 5 and as plasticizer Sugar (i.e. sucrose) or Sorbitolis used. The recipe in example 5 is used as starting formulation towhich additional plasticizer is added resulting in recipes 17 till 20with consequently higher dry solids. By increasing the dry solids withadditional amounts of sucrose or sorbitol the meltability of theformulation remains good. However increasing amounts of sucrose orsorbitol in the formulation lengthens the setting speed (especially thetime needed till 60 cJ).

TABLE 6 Recipe 13 17 18 14 19 20 Plasticizer used Sugar Sugar SugarSorbitol Sorbitol Sorbitol (sucrose) (sucrose) (sucrose) Water (g added) 53.5  51.7  49.4  53.5  51.7  49.4 ETENIA 457 (on dry solids)   4.1  3.9   3.8   4.1   3.9   3.8 MS 1168 (on dry solids)  37.9  36.5  31.5 37.9  36.5  31.5 Plasticizer (g as is)   4.5   7.9  15.3   4.5   7.9 15.3 Directly after preparation Viscosity 80° C. [RVU] 1772 1901 13811478 1754  960 Refraction [%]  50.7  53.0  54.4  50.5  52.7  54.6 Wettack [cJ] [5, 3]  42  36.5*  43  42  39  38 Adhesion Work at [0, 0] [cJ] 34  33  35  34  34  29 Work at [0, 3] [cJ]  46  41  45  43  41.5  38Work at [0, 5] [cJ]  51.5  47.5  48  50  48  44 Setting time till 40 cJ[s] 2-3 2-3  2-3 2-3  2-3  4-5 Setting time till 60 cJ [s] 7-8 9-1012-13 8-9 11-12 14-15 Appearance after 24 hours Hard Hard Hard Hard gelHard gel Hard gel at room temperature gel gel gel After heating at 100°C. Appearance Liquid Liquid Liquid Liquid Liquid Liquid Viscosity 80° C.[RVU] 2107 2999 2429 1706 2306 1355 Wet tack [cJ] [5, 3]  35*  40*  35* 36*  40*  34* Adhesion Work at [0, 0] [cJ]  31.5  33.5  31.5  29  34 29 Work at [0, 3] [cJ]  47.5  44  42  42  43  39 Work at [0, 5] [cJ] 57  50  46.5  46.5  48.5  42 Setting time till 40 cJ [s] 2-3 2-3  2-32-3  2-3  4-5 Setting time till 60 cJ [s] 6-7 9-10 13-14 8-9 11-12  15*Diminished adhesive transfer

The invention claimed is:
 1. Method for adhering a first substrate to asecond substrate comprising the steps of providing an aqueous adhesivecomposition comprising a thermoreversible gelling starch, a bondingstarch, and a plasticizer, wherein the composition has a water contentof at least 30 wt. %; and connecting the first and the second substrateusing the adhesive composition, wherein the adhesive composition is inliquid form; and cooling the adhesive composition, wherein the adhesivecomposition is thermoreversible, wherein the adhesive composition losesits thermoreversibility when the adhesive composition is set by drying.2. Method according to claim 1, wherein the aqueous adhesive compositionis provided in solidified form and subsequently melted to liquid form.3. Method according to claim 1, wherein the adhesive composition iscooled from a temperature of 50° C. or higher to a temperature of 40° C.or lower.
 4. Method according to claim 1, wherein the thermoreversiblegelling starch is obtained from an amylose containing starch, whichstarch has been treated with an enzyme having α-1,4-α-1,4-glucosyltransferase activity.
 5. Method according to claim 1, wherein thethermoreversible gelling starch is obtained by partial debranching of astarch.
 6. Method according to claim 5, wherein the thermoreversiblegelling starch is obtained by treating a starch withα-1,6-D-glucanohydrolase.
 7. Method according to claim 5, wherein thethermoreversible gelling starch comprises up to 80 wt. % short chainamylose and at least 20 wt. % partially debranched amylopectin. 8.Method according to claim 1, wherein the thermoreversible gelling starchcomprises 5-95 wt. % of a crosslinked starch and 95-5 wt. % of adegraded starch.
 9. Method according to claim 1, wherein the bondingstarch is a degraded starch.
 10. Method according to claim 1, whereinthe bonding starch is a low dextrose equivalent maltodextrin having adextrose equivalent of less than
 20. 11. Method according to claim 9,wherein the bonding starch is derived from starch containing more than95 wt. % amylopectin, based on the total starch weight.
 12. Methodaccording to claim 10, wherein the bonding starch is derived from starchcontaining more than 95 wt. % amylopectin, based on the total starchweight.
 13. Method according to claim 1, wherein the plasticizer is alow molecular weight sugar.
 14. Method according to claim 13, whereinthe plasticizer is selected from the group consisting of sorbitol,mannitol and sucrose.
 15. Method according to claim 1, wherein thecontent of the plasticizer in the adhesive composition is 0.1-50 wt. %,based on the total weight of the adhesive composition.
 16. Methodaccording to claim 15, wherein the content of the plasticizer in theadhesive composition is 1-40 wt. %, based on the total weight of theadhesive composition.
 17. Method according to claim 16, wherein thecontent of the plasticizer in the adhesive composition is 3-30 wt %,based on the total weight of the adhesive composition.
 18. Methodaccording to claim 1, comprising 1-60 wt % thermoreversible gellingstarch, based on the total weight of the composition.
 19. Methodaccording to claim 18, comprising 2-40 wt. % thermoreversible gellingstarch, based on the total weight of the composition.
 20. Methodaccording to claim 19, comprising 3-35 wt. % thermoreversible gellingstarch, based on the total weight of the composition.
 21. Methodaccording to claim 1, comprising 5-80 wt. % bonding starch, based on thetotal weight of the composition.
 22. Method according to claim 21,comprising 10-75 wt. % bonding starch, based on the total weight of thecomposition.
 23. Method according to claim 22, comprising 30-70 wt. %bonding starch, based on the total weight of the composition.
 24. Methodaccording to claim 1, comprising 30-80 wt. % water, based on the totalweight of the composition.
 25. Method according to claim 24, comprising30-70 wt. % water based on the total weight of the composition. 26.Method according to claim 1, wherein the composition is athermoreversible gel.
 27. Method according to claim 1, wherein thecomposition is a hotmelt adhesive.
 28. Method according to claim 1,wherein the composition is a drying adhesive.